This invention has to do with circuit arrangments for voice communication, typically from a local station via a radio link or the like to a central station, with provision for generating code tone signals over the same link.
The invention is concerned especially with improving the known type of compact communication unit which may typically be held conveniently in the hand with flexible cable connections to the local station of the radio link. Such a hand-held unit typically includes a small microphone which may normally operate as loud speaker to reproduce incoming audio signals. Operation of a manual switch connects the microphone to an amplifier for outgoing voice transmission. Additionally, a plurality of tone generating circuits having distinctive respective frequencies may be manually actuated to transmit code signals, as to alert the central station.
An important object of the present invention is to permit communication units of the described type to be made more compact and economical. In particular, it is desired to reduce the number of major elements that must be individually mounted on the circuit board of the unit and provided with suitable electrical connections.
The presently available units typically include at least two tone generators with outputs connected in parallel to give a distinctive sound for the code signaling phase of operation. Each tone generator comprises an individual amplifier which is caused to oscillate by positive feedback and is provided with suitable means for stabilizing the frequency of the resulting oscillation. Stabilized oscillators of that general type are available commercially in the form of a two-stage transistor amplifier with the stages coupled by frequency stabilizing means which may comprise a tuned electrical circuit or a mechanical device such as a tuning fork with piezoelectric coupling elements, or the like. For the sake of definiteness the invention will be described primarily as utilizing oscillating amplifiers each formed as an integrated circuit and with frequency stabilization by an external tuning fork coupled to the respective amplifier stages by piezoelectric transducers cemented to the two tines of the fork. With that illustrative arrangement, an integrated circuit and a tuning fork must be provided for each individual tone that is to be generated. Since purchasers usually specify capability for generating a definite number of tones, there is then little or no flexibility in the number of major elements required for the tone generating portion of the equipment.
Communication units of the described type are commonly controlled manually by two distinct switched of push-button type, with the control buttons well separated from each other on the unit to facilitate their separate operation. One switch is operated to enable voice transmission over the radio link. The tone code generating phase is enabled by operating the other switch, or by operating both switches together. With neither switch operated the unit is in idle condition, and the microphone is then typically connected between ground and an audio input terminal so that it can act as loudspeaker to reproduce any incoming communication from the radio link. The described control format requires at least two switches, each of which must have several distinct electrical poles to handle such functions as switching on the radio link if it is normally idle, energizing the microphone amplifier for voice transmission and the tone generating amplifiers for tone production, switching the microphone between its amplifier and the audio input line, and supplying the radio link with the proper output signals for the respective modes of operation. Thus, it is difficult or impossible to reduce significantly the space requirements for the switching facility.